Functional imaging is a term of art that in general refers to techniques in which the aim is to extract quantitative information about physiological function from image-based data. Optical coherence tomography (OCT) is an imaging modality that can resolve much smaller features than ultrasound, and overcomes issues associated with the scatter of visible and near-visible light in human tissue that make other forms of optical imaging quite difficult.
In conventional fiber-based OCT, an interferometer is used to collect image data of a sample under test. FIG. 1 is a block diagram of a prior art apparatus 10 that exemplifies a Michelson interferometer. Light from light source 12 travels through a single-mode optical fiber to beam splitter 17, typically a 50/50 beam splitter. Beam splitter 17 directs a portion of the light along a single-mode optical fiber sample arm to the sample under test 14, with the remainder of the light directed along a single-mode optical fiber reference arm to the mirror 18. Sample under test 14 is, for example, a fiber-optic device or human tissue. The delay line with a mirror 18 increases the optical length of the reference arm. By moving the mirror back and forth, reflection data can be collected at different depths within sample under test 14. Light reflected from sample under test 14 is received by photon detector 16, as is light reflected from the moveable mirror 18. Photon detector 16 has just one spatial channel (that is, a single large pixel).
To obtain an image, the sample arm optical fiber is placed at a location in (or on) the sample. A depth scan is obtained at that location. The optical fiber is then moved to an adjacent location and another depth scan is obtained. The process is repeated laterally across the sample, with a depth scan performed at each lateral location. A scan of one line can be referred to as a transverse scan. To create a two-dimensional image of a sample, transverse scans are performed over the area of the sample.
The process of performing depth scans and transverse scans on a sample can be time-consuming. If each depth scan takes 0.01 seconds and 1000×1000 depth scans are performed across the area of the sample, then approximately three hours are needed to complete the measurements.
En-face imaging with a free-space reflectometer and lamp source provides an approach for speeding up the collection of information. En-face images are planar images of the sample, captured simultaneously using parallel optical channels in the sample arm of a device like apparatus 10. In this case, the beam splitter may be a bulk optic free space beam splitter, and the optical signals may propagate in free space within some or all of the arms rather than in optical fibers. Moving the location of the reflector in the reference arm changes the optical depth at which image information is collected. Image information can be collected in two dimensions simultaneously and thus more rapidly. However, lateral scatter of photons from adjacent sample locations can reduce contrast. Thus, en-face imaging, while speeding up image collection, can reduce image quality.
Accordingly, a functional imaging system and/or method that addresses the problems described above would be of value.